The biosynthesis of the oligosaccharide side chain of aorta glycoproteins is currently under study. Cell free extracts of aorta catalyze the incorporation of mannose from GDP-(C14) mannose and GlcNAc from UDP-(H3)GlcNAc into lipids and into protein. Two different types of lipids are formed from GDP-mannose; one of these lipids is a mannosyl-phosphoryl-polyisoprenol whereas the other is an oligosaccharide-lipid in which various sized oligosaccharides are probably attached to a polyprenol through a pyrophosphoryl linkage. The oligosaccharides released from the lipid by mild acid hydrolysis could be separated into 3 or 4 discrete compounds by paper chromatography. These oligosaccharides had GlcNAc at the reducing end and some mannose in alpha-linkages at the non-reducing end. GlcNAc from UDP-(H3) GlcNAc is also incorporated into lipid but in this case most of the radioactivity is into a compound which has been characterized as GlcNAc-pyrophosphoryl-polyprenol. However if incubations are chased with a large excess of cold UDP-GlcNAc, one can accumulate an N,N'-diacetylchitobiosyl-pyrophosphoryl-lipid. Further, the radioactivity can be chased from this disaccharide-lipid into oligosaccharide-lipids by the addition of unlabeled GDP-mannose. These various reactions as well as synthesis of the glycoproteins are being studied in detail.